The present invention relates to a drive system including a secondary axle drive module which is selectively engageable to create an overspeed condition wherein torque may be transferred to the secondary axle even when the secondary axle is rotating faster than the primary axle.
Vehicle drive lines typically require a torque path to each driven wheel, which is usually a propeller shaft from a power take-off or transmission to a differential, which distributes power to the wheels on each axle. For acceptable steering and efficiency, a vehicle drive line must allow each wheel speed to be different. In normal vehicles with only the front wheels steered, each wheel follows a different path in cornering, with a different radius. This relationship is illustrated in FIG. 1. As shown, each wheel follows a different radial path.
If two wheels on an axle are to rotate at minimal slip, they cannot be coupled in such a way that they must be rotate at the same speed. An axle differential provides power to both wheels while allowing them to rotate at different speeds. If both axles are to be powered, then the average axle speed differences must also be considered. Since the front wheels follow a larger average radius than the rear wheels, locking the axles together results in slip while turning. This is particularly problematic on surfaces having a high frictional coefficient (xcexc), such as asphalt or concrete. Operating a vehicle with axles locked together on a high (xcexc) surface results in unacceptable binding and steering reactions, along with substantial drive line and tire wear. Thus, vehicles that must power both axles in such conditions require a center device that sends power to both axles while allowing variation in speed (such as with a differential or coupling).
Typical prior art drive line arrangements include two wheel drive (traditional front or rear wheel drive), part-time four wheel drive, permanent four wheel drive (all-wheel drive) with center differential, and permanent four wheel drive (all-wheel drive) with coupling. These systems have their advantages and disadvantages, but a standard limitation is that these systems do not allow full torque to be applied to the secondary axle when it is spinning faster than the primary axle when both are on surfaces with similar (xcexc). In the case of an all-wheel drive (AWD) vehicle with a center coupling, the front/rear torque distribution split is limited by the load distribution of the vehicle.
It would be desirable to provide a drive system which enables complete front/rear torque control, independently of axle speeds.
The present invention provides a vehicle drive system which allows torque to be selectively applied to a secondary axle under a wider range of wheel speed conditions than is possible with prior art four wheel drive system, while retaining the, capability of efficient torque transfer under nominal conditions, i.e. when wheel speeds are nearly equal. The drive system of the invention allows torque to be applied to the secondary axle even when the secondary axle is spinning faster than the primary axle, thus enabling greater range of torque split between the front and rear axles.
More specifically, the invention provides a vehicle drive system including a primary axle driven by torque from a transmission, an output shaft driven by the primary axle or transmission, a secondary axle, a secondary axle clutch configured to operatively connect the secondary axle with the output shaft, and a secondary axle drive module. The secondary axle drive module is operatively connected between the output shaft and the secondary axle. The secondary axle drive module includes an overspeed gear set connected in parallel with a one-way clutch to facilitate application of torque to the secondary axle even when the secondary axle is rotating faster than the primary axle.
The overspeed gear set is selectively engageable with the secondary axle through an overspeed clutch. A secondary axle differential is operatively engaged between the secondary axle drive module and the secondary axle.
An angle gear set is operatively connected between the output shaft and the drive module. The overspeed gear set includes first and second intermeshed gears forming a first subset, and third and fourth intermeshed gears forming a second subset, wherein the first gear is connected to the angle gear set, the second gear is affixed to the third gear, and the fourth gear is selectively engageable with the secondary axle through an overspeed clutch. The first, second, third and fourth gears are sized and arranged so that the fourth gear rotates faster than the first gear.
The angle gear set, drive module, and secondary clutch may be arranged in any order sequentially in the torque flow path between the output shaft and secondary axle.
The secondary axle drive module enables torque distribution adjustment between 100% primary axle drive and 100% secondary axle drive. The overspeed gear set is configured to selectively rotate the secondary axle approximately 20% faster than the primary axle.
In an alternative embodiment, left and right clutches are provided on the secondary axle to selectively engage and disengage left and right wheels to allow control of the torque split side to side at the secondary axle.